Loudspeakers with panel-form acoustic radiating elements

ABSTRACT

A loudspeaker ( 81 ) comprising an enclosure, an acoustic radiator ( 2 ) in the enclosure, a compliant suspension (3) mounting the radiator in the enclosure for pistonic movement relative thereto, and transducer means ( 9 ) for driving the radiator pistonically, characterized in that the radiator is a panel-form distributed mode acoustic radiator, by a first transducer mounted wholly and exclusively on the radiator to vibrate the radiator to cause it to resonate, and by means ( 11 ) for varying the air pressure in the enclosure to cause the radiator to move pistonically.

TECHNICAL FIELD

[0001] The invention relates to loudspeakers and more particularly toloudspeakers comprising panel-form acoustic radiating elements.

BACKGROUND ART

[0002] It is known from GB-A-2262861 to suggest a panel-form loudspeakercomprising:

[0003] resonant multi-mode radiator element being a unitary sandwichpanel formed of two skins of material with a spacing core of transversecellular construction, wherein the panel is such as to have ratio ofbending stiffness (B), in all orientations, to the cube power of panelmass per unit surface area (μ) of at least 10;

[0004] a mounting means which supports the panel or attaches to it asupporting body, in a free undamped manner;

[0005] and an electromechanical drive means coupled to the panel whichserves to excite a multi-modal resonance in the radiator panel inresponse to an electrical input within a working frequency band for theloudspeaker.

DISCLOSURE OF INVENTION

[0006] Embodiments of the present invention use members of nature,structure and configuration achievable generally and/or specifically byimplementing teachings of our co-pending PCT application no. (our caseP.5711) of even date herewith. Such members thus have capability tosustain and propagate input vibrational energy by bending waves inoperative area(s) extending transversely of thickness often but notnecessarily to edges of the member(s); are configured with or withoutanisotropy of bending stiffness to have resonant mode vibrationcomponents distributed over said area(s) beneficially for acousticcoupling with ambient air; and have predetermined preferential locationsor sites within said area for transducer means, particularlyoperationally active or moving part(s) thereof effective in relation toacoustic vibrational activity in said area(s) and signals, usuallyelectrical, corresponding to acoustic content of such vibrationalactivity. Uses are envisaged in co-pending International application No.(our file P.5711) of even date herewith for such members as or in“passive” acoustic devices without transducer means, such as forreverberation or for acoustic filtering or for acoustically “voicing” aspace or room; and as or in “active” acoustic devices with transducermeans, such as in a remarkably wide range of sources of sound orloudspeakers when supplied with input signals to be converted to saidsound, or in such as microphones when exposed to sound to be convertedinto other signals.

[0007] This invention is particularly concerned with active acousticdevices in the form of loudspeakers.

[0008] Members as above are herein called distributed mode acousticradiators and are intended to be characterised as in the above PCTapplication and/or otherwise as specifically provided herein.

[0009] The invention provides a loudspeaker comprising an enclosure, anacoustic radiator in the enclosure, a compliant suspension mounting theradiator in the enclosure for limited pistonic movement relativethereto, and transducer means for driving the radiator, characterised inthat the radiator is a panel-form distributed mode acoustic radiator, bya first transducer mounted wholly and exclusively on the radiator tovibrate the radiator to cause it to resonate, and by means for varyingthe air pressure in the enclosure to cause the radiator to movepistonically. The air pressure varying means may comprise an air pump.The air pump may comprise a subsidiary enclosure, a pistonic drivermounted in the subsidiary enclosure and means coupling the interiors ofthe respective enclosures such that air pressure waves produced bymotion of the pistonic driver are transmitted to the said enclosure.

[0010] Acoustically absorbent means, e.g. wadding, may be provided inthe said enclosure and/or in the subsidiary enclosure.

[0011] The distributed mode acoustic radiator may comprise a panelhaving a lightweight cellular core sandwiching a pair of high moduluslightweight skins.

BRIEF DESCRIPTION OF DRAWINGS

[0012] The invention is diagrammatically illustrated, by way of example,in the accompanying drawings, in which:

[0013]FIG. 1 is a diagram showing a distributed-mode loudspeaker asdescribed and claimed in our co-pending International application No.(our case P.5711);

[0014]FIG. 2a is a partial section on the line A-A of FIG. 1;

[0015]FIG. 2b is an enlarged cross-section through a distributed moderadiator of the kind shown in FIG. 2a and showing two alternativeconstructions, and

[0016]FIG. 3 is a diagram of an embodiment of distributed-modeloudspeaker according to the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

[0017] Referring to FIG. 1 of the drawings, there is shown a panel-formloudspeaker (81) of the kind described and claimed in our co-pendingInternational application No. (our case P.5711) of even date herewithcomprising a rectangular frame (1) carrying a resilient suspension (3)round its inner periphery which supports a distributed mode soundradiating panel (2). A transducer (9) e.g as described in detail withreference to our co-pending International applications Nos. (our casesP.5683/4/5) of even date herewith, is mounted wholly and exclusively onor in the panel (2) at a predetermined location defined by dimensions xand y, the position of which location is calculated as described in ourco-pending International application No. (our case P.5711) of even dateherewith, to launch bending waves into the panel to cause the panel toresonate to radiate an acoustic output.

[0018] The transducer (9) is driven by a signal amplifier (10), e.g. anaudio amplifier, connected to the transducer by conductors (28).Amplifier loading and power requirements can be entirely normal, similarto conventional cone type speakers, sensitivity being of the order of86-88 dB/watt under room loaded conditions. Amplifier load impedance islargely resistive at 6 ohms, power handling 20-80 watts. Where the panelcore and/or skins are of metal, they may be made to act as a heat sinkfor the transducer to remove heat from the motor coil of the transducerand thus improve power handling.

[0019]FIGS. 2a and 2 b are partial typical cross-sections through theloudspeaker (81) of FIG. 1. FIG. 2a shows that the frame (1), surround(3) and panel (2) are connected together by respective adhesive-bondedjoints (20). Suitable materials for the frame include lightweightframing, e.g. picture framing of extruded metal e.g. aluminium alloy orplastics. Suitable surround materials include resilient materials suchas foam rubber and foam plastics. Suitable adhesives for the joints (20)include epoxy, acrylic and cyano-acrylate etc. adhesives.

[0020]FIG. 2b illustrates, to an enlarged scale, that the panel (2) is arigid lightweight panel having a core (22) e.g. of a rigid plastics foam(97) e.g. cross linked polyvinylchloride or a cellular matrix (98) i.e.a honeycomb matrix of metal foil, plastics or the like, with the cellsextending transversely to the plane of the panel, and enclosed byopposed skins (21) e.g. of paper, card, plastics or metal foil or sheet.Where the skins are of plastics, they may be reinforced with fibres e.g.of carbon, glass, Kevlar (RTM) or the like in a manner known per se toincrease their modulus.

[0021] Envisaged skin layer materials and reinforcements thus includecarbon, glass, Kevlar (RTM), Nomex (RTM) i.e. aramid etc. fibres invarious lays and weaves, as well as paper, bonded paper laminates,melamine, and various synthetic plastics films of high modulus, such asMylar (RTM), Kaptan (RTM), polycarbonate, phenolic, polyester or relatedplastics, and fibre reinforced plastics, etc. and metal sheet or foil.Investigation of the Vectra grade of liquid crystal polymerthermoplastics shows that they may be useful for the injection mouldingof ultra thin skins or shells of smaller size, say up to around 30 cmdiameter. This material self forms an orientated crystal structure inthe direction of injection, a preferred orientation for the goodpropagation of treble energy from the driving point to the panelperimeter.

[0022] Additional such moulding for this and other thermoplastics allowsfor the mould tooling to carry location and registration features suchas grooves or rings for the accurate location of transducer parts e.g.the motor coil, and the magnet suspension. Additional with some weakercore materials it is calculated that it would be advantageous toincrease the skin thickness locally e.g. in an area or annulus up to150% of the transducer diameter, to reinforce that area and beneficiallycouple vibration energy into the panel. High frequency response will beimproved with the softer foam materials by this means.

[0023] Envisaged core layer materials include fabricated honeycombs orcorrugations of aluminium alloy sheet or foil, or Kevlar (RTM), Nomex(RTM), plain or bonded papers, and various synthetic plastics films, aswell as expanded or foamed plastics or pulp materials, even aerogelmetals if of suitably low density. Some suitable core layer materialseffectively exhibit usable self-skinning in their manufacture and/orotherwise have enough inherent stiffness for use without laminationbetween skin layers. A high performance cellular core material is knownunder the trade name ‘Rohacell’ which may be suitable as a radiatorpanel and which is without skins. In practical terms, the aim is for anoverall lightness and stiffness suited to a particular purpose,specifically including optimising contributions from core and skinlayers and transitions between them.

[0024] Several of the preferred formulations for the panel employ metaland metal alloy skins, or alternatively a carbon fibre reinforcement.Both of these, and also designs with an alloy Aerogel or metal honeycombcore, will have substantial radio frequency screening properties whichshould be important in several EMC applications. Conventional panel orcone type speakers have no inherent EMC screening capability.

[0025] In addition the preferred form of piezo and electro dynamictransducers have negligible electromagnetic radiation or stray magnetfields. Conventional speakers have a large magnetic field, up to 1 meterdistant unless specific compensation counter measures are taken.

[0026] Where it is important to maintain the screening in anapplication, electrical connection can be made to the conductive partsof an appropriate DML panel or an electrically conductive foam orsimilar interface may be used for the edge mounting.

[0027] The suspension (3) may damp the edges of the panel (2) to preventexcessive edge movement of the panel. Additionally or alternatively,further damping may be applied, e.g. as patches, bonded to the panel inselected positions to damp excessive movement to distribute resonanceequally over the panel. The patches may be of bitumen-based material, ascommonly used in conventional loudspeaker enclosures or may be of aresilient or rigid polymeric sheet material. Some materials, notablypaper and card, and some cores may be self-damping. Where desired, thedamping may be increased in the construction of the panels by employingresiliently setting, rather than rigid setting adhesives.

[0028] Effective said selective damping includes specific application tothe panel including its sheet material of means permanently associatedtherewith. Edges and corners can be particularly significant fordominant and less dispersed low frequency vibration modes of panelshereof. Edge-wise fixing of damping means can usefully lead to a panelwith its said sheet material fully framed, though their corners canoften be relatively free, say for desired extension to lower frequencyoperation. Attachment can be by adhesive or self-adhesive materials.Other forms of useful damping, particularly in terms of more subtleeffects and/or mid- and higher frequencies can be by way of suitablemass or masses affixed to the sheet material at predetermined effectivemedial localised positions of said area.

[0029] An acoustic panel as described above is bi-directional. The soundenergy from the back is not strongly phase related to that from thefront. Consequently there is the benefit of overall summation ofacoustic power in the room, sound energy of uniform frequencydistribution, reduced reflective and standing wave effects and with theadvantage of superior reproduction of the natural space and ambience inthe reproduced sound recordings.

[0030] While the radiation from the acoustic panel is largelynon-directional, the percentage of phase related information increasesoff axis. For improved focus for the phantom stereo image, placement ofthe speakers, like pictures, at the usual standing person height,confers the benefit of a moderate off-axis placement for the normallyseated listener optimising the stereo effect. Likewise the triangularleft/right geometry with respect to the listener provides a furtherangular component. Good stereo is thus obtainable.

[0031] There is a further advantage for a group of listeners comparedwith conventional speaker reproduction. The intrinsically dispersednature of acoustic panel sound radiation gives it a sound volume whichdoes not obey the inverse square law for distance for an equivalentpoint source. Because the intensity fall-off with distance is much lessthan predicted by inverse square law then consequently for off-centreand poorly placed listeners the intensity field for the panel speakerpromotes a superior stereo effect compared to conventional speakers.This is because the off-centre placed listener does not suffer thedoubled problem due to proximity to the nearer speaker; firstly theexcessive increase in loudness from the nearer speaker, and then thecorresponding decrease in loudness from the further loudspeaker.

[0032] There is also the advantage of a flat, lightweight panel-formspeaker, visually attractive, of good sound quality and requiring onlyone transducer and no crossover for a full range sound from each paneldiaphragm.

[0033]FIG. 3 illustrates another way of combining pistonic anddistributed mode resonant behaviour in a loudspeaker (81). In thedrawing a lightweight, rigid distributed mode sound radiator panel (2)of the kind shown in FIGS. 1 and 2 forms a front wall of a box-likeenclosure (8) having sides (135) and a rear wall (12) e.g. of mediumdensity fibreboard, together defining a cavity (155). A panel (51) ofacoustic absorption material is provided in the cavity (155). A panel(51) of acoustic absorption material is provided in the cavity to dampstanding waves. The radiator panel (2) is mounted in the enclosure (8)by means of a compliant suspension (7) e.g. to emulate the roll surroundof a conventional pistonic cone loudspeaker and carries a transducer (9)of the kind described with reference to our co-pending Internationalapplication Nos. (our files (P5683/4/5) of even date herewith mountedwholly and exclusively on the panel (2) at a predetermined location asdescribed in our said co-pending International application No. (our fileP.5711) of even date herewith to launch bending waves into the panel.

[0034] The interior cavity (155) of the enclosure (8) is coupled to abass pump (11), that is to say to the interior of a box-like enclosure(185) containing a pistonic bass loudspeaker drive unit (42), by meansof a pipe-like conduit (90), whereby air pressure waves of acousticfrequency in the bass region are applied to the interior (155) of theenclosure to cause the panel (2) to move pistonically on its compliantsuspension (7) to produce a low frequency acoustic output. In additionthe panel is caused to resonate by the transducer (9) to cause the panelto radiate an acoustic output at higher frequencies. An amplifier (1) isarranged to feed an acoustic signal to the bass pump (11) and to thetransducer (9) to drive the loudspeaker.

1. A loudspeaker comprising an enclosure, an acoustic radiator in theenclosure, a compliant suspension mounting the radiator in the enclosurefor pistonic movement relative thereto, and transducer means for drivingthe radiator pistonically, characterised in that the radiator is apanel-form distributed mode acoustic radiator, by a first transducermounted wholly and exclusively on the radiator to vibrate the radiatorto cause it to resonate, and by means for varying the air pressure inthe enclosure to cause the radiator to move pistonically.
 2. Aloudspeaker according to claim 1 , characterised in the air pressurevarying means comprises an air pump.
 3. A loudspeaker according to claim2 , characterised in that the air pump comprises a subsidiary enclosure,a pistonic driver mounted in the subsidiary enclosure and means couplingthe interiors of the respective enclosures such that air pressure wavesproduced by motion of the pistonic driver are transmitted to the saidenclosure.
 4. A loudspeaker according to claim 3 , characterised byacoustically absorbent means in the said enclosure and/or in thesubsidiary enclosure.
 5. A loudspeaker according to any preceding claim,characterised in that the distributed mode acoustic radiator comprises apanel having a lightweight cellular core sandwiching a pair of highmodulus lightweight skins.